Four interrelated but independent hypotheses are being tested in this proposal. A) Aptamers can serve as molecular probes to identify functionally important exosites on coagulation factor proteases, B) Exosite-binding aptamers that inhibit coagulation factors in the contact pathway can limit thrombosis without increasing bleeding, C) Combinations of two exosite-binding aptamers and combinations of an aptamer and small molecule active site inhibitor represent potent, yet rapidly reversible anticoagulation strategies that can support cardiopulmonary bypass surgery and D). The exosite binding Factor IX/IXa aptamer targeting a contact pathway factor will limit factor Xa and thrombin generation and inflammation more effectively than targeting common pathway factors in patients undergoing PCI. Each of these lines of investigation rationally build upon important results obtained in the current funding cycle of this award. Our specific aims are: Aim 1: To utilize aptamers to identify exosites on coagulation factors XIIa, XIa, IXa, Xa, VIIa and Kallikrein. Aim 2: To evaluate the ability of aptamers targeting exosites on contact pathway factors to act as potent yet safe antithrombotic agents. Aim 3: To elucidate the mechanism by which combinations of A) aptamer-based inhibitors and B) aptamer-based exosite and active site inhibitors of Factor Xa and thrombin synergize and determine if such combinations can produce rapid and safe anticoagulation for cardiopulmonary bypass (CPB) and if antidotes can produce rapid and safe neutralization of anticoagulation following discontinuation of CPB. Aim 4: To determine if our factor IXa aptamer limits thrombin and factor Xa generation more effectively than bivalirudin in ACS patients undergoing PCI and if this results in a reduction in inflammation in such patients.